We have sand and fibre mix. It's been down 2 years and still looks brand new. Rides brilliantly. We regularly harrow it otherwise the fibre does come to the top. Plus we've just rolled it for the first time.
I wouldn't turnout for long though, ours go for a leg stretch if they have had to stay in, both they roll and dig it up if left any longer. Plus I'd be worried about feeding them in there, would hate to see dropped hay:Haylage in it.